In UMTS WCDMA there is a mechanism defined for Random Access transmission in the uplink. The User Equipment UE (namely the mobile station) transmits a randomly-selected preamble signal characterised by:
Signature sequence (i.e. bit sequence)
Scrambling code,
Sub-channel (i.e. timing of the access slot in the frame)
If the base station receives the signature it acknowledges it on the Acquisition Indicator Channel (AICH). If the UE receives a positive acknowledgement it transmits a message part on a RACH data channel.
If the UE does not receive an acknowledgement it can make another attempt. The probability of collision depends on the number of Signature sequences, Scrambling codes, Access slots from which the UEs make their selection for the randomly-selected preamble signal.
Currently in 3GPP it is proposed that UEs can be configured to use a new mechanism, which is similar to RACH. For the purpose of this invention, the main difference is that the data transmission uses a high-speed uplink packet data channel, known as Enhanced Dedicated Channel (E-DCH), instead of a single short message associated directly with the RACH preamble. Unlike the RACH message, the E-DCH is characterised by power control, rate control, hybrid ARQ and the ability to transmit high data rates for an indefinite length of time, but in practice the detailed characteristics are not relevant to this invention.
For convenience we can refer to the existing RACH as “R99 RACH” and the new scheme as “E-RACH”.
The resources from which the UE is permitted to make a random selection for the R99 RACH preamble signal transmission are broadcast on a broadcast channel (BCH) in each cell. The R99 RACH parameters which are signalled on the BCH are included for reference in the table below. This information is provided for one of more PRACHs (Physical RACHs):
InformationElement/GroupTypenameNeedMultiand referenceSemantics descriptionVersion>>AvailableMPBit stringEach bit indicatesSignature(16)availability for asignature, where thesignatures are numbered“signature 0” up to“signature 15”. The value1 of a bit indicates thatthe correspondingsignature is available andthe value 0 that it is notavailable.>>PreambleMPIntegerIdentification ofscrambling code(0 . . . 15)scrambling code]number>>Available SubMPBit stringEach bit indicatesChannel Number(12)availability for asubchannel, where thesubchannels arenumbered “subchannel 0”to “subchannel 11”. Thevalue 1 of a bit indicatesthat the correspondingsubchannel is availableand the value 0 indicatesthat it is not available.
This information comprises a relatively large number of bits which have to be transmitted with sufficient power to be received throughout the cell. This represents a high overhead.